Based on the huge business opportunities brought by convenience of digitalization, electrical circuit and sensor are integrated on many traditional tools to display related data when using the tools. When using a traditional torque tool, such as a torque wrench, the extent of tightness of locking only can be determined by the user's experience and can not be judged by a reliable quantified data. Therefore, some wrenches with indication needles made by machinery, by which the user can see the quantity of the force exerted, appear in the market. However, the data shown on the indication needle sometimes can not be read easily and lacks flexibility of calibration. Further, records and statistics of the data can not be applied to match with digitalized integration thus the electronic and digitized tools are developed. The followings are some conventional electronic and digitized tools: (1) U.S. Pat. No. 3,970,155 disclosing an electronic torque wrench, in which a pair of strain gauges are attached with the torque wrench and a bridge circuit is electrically connected with the strain gauges such that the variations of the strain gauges can be measured by the bridge circuit. (2) U.S. Pat. No. 4,976,133 disclosing an apparatus for adjusting and calibrating a torque wrench, in which strain gauges attached with the torque wrench are also utilized to detect the force exerted on the torque wrench. (3) Taiwan Patent Publication No. 520,321 disclosing a wrench, in which strain gauges are arranged on both sides of the wrench and an integration device is arranged on the center of the wrench, the integration device is electrically connected with the strain gauges so as to detect strain variations of the strain gauges.
To sum up the above-mentioned prior arts, general drawbacks existed therein are described in the following. (a) Referring to FIG. 1, which is a diagram showing a prior art for measuring deform of main body of a torque tool. For example, a strain gauge 11a is attached on the main body of a wrench 1a to measure strain amount of the wrench. A strain variation is generated when a force is exerted on the wrench 1a, and a strain or a deform is also generated on the strain gauge 11b of the deformed wrench 1b. According to the deform or displacement of the strain gauge 11b, the force exerted on the wrench can be known. According to FIG. 1, the strain amount ε=Mc/EI, wherein ε is strain amount of the wrench, M is torque exerted on the wrench tool, c is distance from central axis to surface of the wrench, E is Young's modulus of the wrench, I is momentum of inertia of the wrench. Since the momentum of inertia and the Young's modulus of the wrench is large due to strengthened structural design for main body of the wrench tool it is hard to achieve a large strain amount ε. (b) Two or four strain gauges are required to be attached on the main body of the wrench tool, thus the cost is increased and the production efficiency is decreased. (c) A complicated analog/digital circuit is utilized for controlling.
In view of the above-mentioned problems, an improved apparatus for measuring torque of a torque tool by using an indirect structure is required to solve the defects of the prior arts.